For an image display device that displays gradient images, the required range of gradation is usually selected from the range of gradation for the image data, and is displayed after being matched to a fixed range of gradation for the display unit. For example, an image display unit used for computer tomography can accommodate image data having gradation ranges from -1000 to +3000. A gradation range (called a window and level) that displays the states of individual sections most appropriately is selected according to the desired sections, and is displayed within the windows and levels of fixed gradation for the display unit. A gradation conversion circuit is built into the display device to match the selected range of image data gradation to that of the display device. Such image display devices as described above have conventionally required multiple gradation conversion circuits, which is not desirable due to increased hardware requirements when an operator wishes to display multiple sections on the same screen with each section displayed in a discrete range of gradation.